The present invention relates to a compound roll composed of a shell and a core, and more particularly to an adamite compound roll composed of a high-strength, high-toughness, low-alloy cast iron or steel shell and a cast or forged steel core.
Rolls for hot and cold rolling are conventionally formed from alloy cast iron or hardened forged steel, but they suffer from various problems such as low resistance to wear and failure. For the purpose of improving the toughness of rolls, adamite rolls were developed.
Due to the recent advancement of steel rolling, rolls have been used under increasingly severer conditions. Thus, the adamite rolls were sometimes forged to disperse carbides and to make matrix grains finer. The forging is effective for improving the strength and toughness of the adamite rolls, but the forged adamite rolls are expensive, and the forging still fails to attain sufficient strength at neck portions of the rolls. To improve the strength of the neck portions too, low-carbon adamite actually had to be used with the wear resistance of roll bodies sacrificed.
Recently, a centrifugal casting method was attempted to manufacture adamite compound rolls. See J. Honda et al., "Compound Cast Rolls for Steel Rolling Mills," IMONO (Casting), Vol. 54, pp. 44-50, 1982. However, the centrifugal casting method has turned out to be unsatisfactory, because it failed to provide compound rolls having sufficiently hard shells and sufficiently tough cores. Specifically, adamite compound rolls manufactured by the centrifugal casting method had cast cores, so that the cores' mechanical properties were lower than required.
In addition, conventional four high mills comprising a pair of work rolls and a pair of back-up rolls have been increasingly replaced by six high mills having intermediate rolls between work rolls and back-up rolls, or mills having work rolls which can be shifted, in order to apply higher pressure to metal sheets to be rolled. Because an extremely high load is applied to the work rolls, the maximum contact pressure of the work rolls can reach, for instance, up to 240 kg/mm.sup.2 as compared with 160 kg/mm.sup.2 for the four high mills. At the same time, in such high-pressure mills, a larger bending force is applied to the shafts of the work rolls, so that the roll shafts have been required to have high mechanical strength.
In view of the above problems with the centrifugal casting method, a method of forming a shell around a core by casting a shell material around the core was recently developed.
U.S. Pat. No. 3,455,372 issued to Yamamoto on July 15, 1969 discloses a continuous padding method using high frequency current. This method comprises preheating the surface of a core material by moving the core material up and down through a mold assembly composed of a heating mold, a buffer mold and a cooling mold, and after returning the core material to a predetermined position, moving it downwardly and slowly through the mold assembly while pouring a melt of padding material into the gap between the core material and the mold assembly, whereby the melt is bonded to the surface of the core material, cooled to some extent within the buffer mold, and further cooled and solidified rapidly within the cooling mold to form a layer of pad on the surface of the core material.
This method, which may be called simply "shell casting method," can provide a compound roll composed of a hard shell and a tough core. Adamite compound rolls manufactured by the shell casting method are subject to heat treatment. However, a usual heat treatment comprising hardening and tempering fails to achieve the maximum properties which these compound rolls potentially have. Particularly, cracking which may lead to breakage of roll bodies remains to be a serious problem for the adamite compound rolls thus manufactured.